Ch.21&22: Magnetism and Magnetic Fields.

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“Field Lines Always Point Away from the _____ and Toward the _____.”

NorthSouth

Words to Inspire Us. “The search for truth is more precious than its possession.” - Albert Einstein

Magnetite: From Magnesia (Greece).

Formula: Fe3O4.

The name probably comes from Magnesia, but there is a fable of Magnes, a Greek shepherd, who discovered magnetite when the nails in his shoes stuck to the ground!

Description: Dark grey, slightly shiny. Magnetite is naturally magnetic. It is also called Lodestone. In Middle Ages, pilots were called lodesmen. The lodestar is the Polar star, the leading star by which mariners are guided.

Early History600 BC - LodestoneThe magnetic properties of natural ferric ferrite (Fe3O4) stones (lodestones) were described by Greek philosophers.

1175 - First Reference to a CompassAlexander Neckem an English monk of St. Albans describes the workings of a compass.1269 - First Detailed Description of a CompassPetrus Peregrinus de Marincourt, a French Crusader, describes a floating compass and a compass with a pivot point.

1040 - One of the earliest compasses (China)A floating fish-shaped iron leaf, mentioned in the Wu Ching Tsung Yao which was written around 1040. The book describes how iron can be heated and quenched to produce thermo-induced magnetization. The first clear account of suspended magnetic compasses in any language was written by Shen Kua in 1088.

1600 - Static Electricity (De Magnete)William Gilbert(1544-1603) studied magnetism and in 1600 wrote "De magnete" which gave the first rational explanation to the mysterious ability of the compass needle to point north-south: the Earth itself was magnetic. "De Magnete" opened the era of modern physics and astronomy and started a century marked by the great achievements of Galileo, Kepler, Newton and others.

Gilbert recorded three ways to magnetize a steel needle: by touch with a loadstone; by cold drawing in a North-South direction; and by exposure for a long time to the Earth's magnetic field while in a North-South orientation.

The Magnetic FieldThe Magnetic Field

Magnets in Ancient TimesMagnetism has been known since ancient times because it occurs naturally in loadstone, a rock rich in magnetite, a form of iron oxide.

Some Chinese cities are laid out along the direction of the Earth’s magnetic field.

It was believed by some that magnetic fields permeated humans and their manipulation could affect health.

The first compasses were made in China in ~1000 AD.

Sir William Gilbert (1544-1603)

Magnets have two poles, which he called north and south.Like poles repel and opposite poles attract.

Iron can be magnetized.

The Earth is a giant magnet.

Gilbert’s book, De Magnete, was enormously popular and influenced Kepler and Galileo.

The Magnetic FieldThe Magnetic FieldThe ‘Gilbert Model’

Like poles repel, and unlike poles attract.Cut a magnet in half and you will have two magnets.A single pole (monopole) has never been isolated.

Magnetic Field of a Bar Magnet.Field lines always point away from the North and toward the South.

Filing demonstration of magnetic field lines.

Edmond Halley, 1656-1742, (of comet fame) ingeniously proposed that the Earth contained a number of spherical shells, one inside the other, each magnetized differently, each slowly rotating in relation to the others.

He was born in the German town of Iznang. At the age of 32, he completed his medical training at the University of Vienna with a dissertation on the influence of magnetism on human disease.

Franz Anton Mesmer (1734-1815)

•Until 1820, the only magnetism known was that of iron magnets and of "lodestones", natural magnets of iron-rich ore.

•This was changed by a professor of Physics at University of Copenhagen, Hans Christian Oersted (1777-1851).                        

1820 - Electromagnetism, CurrentIn 1820, a physicist Hans Christian Oersted, learned that a current flowing through a wire would move a compass needle placed beside it. This showed that an electric current produced a magnetic field.

The Magnetic FieldThe Magnetic Field

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Oersted’s Compass Deflections

•Andre Ampere – 1775-1836, French scientist.

•Furthered the work of Oersted on the relationship between electricity and magnetism.

•The basic unit of electric current is named after him (Ampere or Amp).

Magnetic Domains - a cluster of magnetically-aligned atoms.

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Magnetic Domains

Not Magnetic Magnetic

Electron Spin in the source of an elements magnetic property.

Michael Faraday (1791-1867) was a British scientist who contributed to the field of electromagnetics.

1820 –Faraday observed Oersted’s compass needle move and wrote, “Use magnetism to produce electricity.”

1831 - Faraday built two devices to produce what he called electromagnetic rotation: the electric motor, t hat used continuous circular motion from the circular magnetic force around a wire.

1832 - The electric generator used a magnet to generate electricity.

GeographicNorth Pole

MagneticNorth Pole

Earth’s Magnetic Field

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Magnetic Field of Earth

The Magnetic Field of the EarthThe Magnetic Field of the Earth

Variations in Compass Deflections

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Van Allen Radiation Belts in Earth’s Magnetic Field

                                                                              

Effect of Solar Wind on Earth’s Magnetic Field

The Right Hand Rule for Wires.

B = μoI / 2πr

μo= 4πx10-7 Tm/A

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The First Right-Hand-Rule

B = μoI/(2πa)

Magnetic Field Generated by a Coil

B magnetic field strength N/(Ampere meter)I current in wire (Amperes)n number of turns of wireL length of coil (meters)

B = μonI/L

μo= 4πx10-7 Tm/A

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Loose Coil

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A Solenoid

B = μonI

N

MAGNETIC FIELD OF A COIL

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The Toroidal Solenoid B = μoNI/(2πr)

The magnitude of the magnetic force is

F = q V B ,

where q is the magnitude of the charge of the particle, V its velocity, and B is the magnetic field. This force can be also considered as the centripetal force

Fc = m v2 / R ,

where m is the particle's mass and R is the radius of the circular trajectory.

Force on a Charged Particle moving in a Magnetic Field.

F = B I L• B is the external magnetic field measured in N/Am.

• I is the current measured in amps.

• L is the length of the current segment inside of the magnetic field, B.

Force on a Current-carrying Wire in a Magnetic Field.

1820 - Andre Marie Ampere showed that two parallel wires carrying current attracted each other if the currents are in the same direction and repelled if the currents are in opposite directions.

He formulated in mathematical terms, the laws that govern the interaction of currents with magnetic fields in a circuit and as a result of this the unit of electric current, the amp, was derived from his name.

F/L=(μoI1I2)/(2πa)

Application: MAGLEV Trains.

Types of Magnetism •Diamagnetism

–opposes the applied field due to electron cloud interaction (Lenz's Law). Characteristic of all materials - Very weak response (k negative).

•Paramagnetism–linear response to applied field, but no residual magnetism when field is removed. Characteristic of Fe, Mn, Co, Ni minerals. Weak; k positive.,

•Ferromagnetism–total alignment of magnetic moments. Usually found in ferrous metals such as steel. Very strong.

•Ferrimagnetism – partial alignment of magnetic moments due to arrangement of ferric and ferrous ions in a lattice structure such as magnetite. Strong (k non-linear).

Anti-ferrimagnetism–Similar to ferrimagnetism, but magnetization opposes applied field. Due to defects (e.g. pyrrhotite) or distorted (canted) lattice structures (e.g., hematite)

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Force on a Current Loop